The invention relates to a fastening device and to a method for producing an integrated fastening structure on or in a sandwich component with a fastening device of this type. In the context of the present invention, sandwich components are multiple-layer components with at least one “soft” core layer and an upper and lower cover layer with higher strength than the core layer, preferably produced from a fiber reinforced material.
The invention relates, in particular, to a snap-together fastening device for plug-in mounting into an opening of a sandwich component, such as a carbon fiber reinforced plastic sandwich component. The snap together fastening device includes two snap-together elements. Fixing arms, which can be actuated from a mounting position into a fixing position, are arranged on one element, which fixing arms penetrate mechanically into the core of the sandwich component. The result is that a non-positive connection to the sandwich component is produced.
Sandwich components, such as carbon fiber reinforced plastic sandwich panels, are increasingly used in the preparation of constructions or in automotive engineering and the like. A maximum of flexural stiffness with simultaneous weight saving can be achieved by way of sandwich panels. As a result of their design, firstly a great stiffness can be achieved with secondly a comparatively low weight. Sandwich panels are available in a very wide range of design variants. As a rule, sandwich panels consist of two cover layers which lie on the outside and a core which lies in between. Thus, for example, a carbon fiber epoxy sandwich panel consists of two cover layers made from carbon fiber fabric with an epoxy resin and a core made from PVC rigid foam.
There is fundamentally a requirement to connect sandwich components to one another or to other components or to fasten parts and elements thereto. The introduction of force for producing connecting points causes a large problem in practice, however. Unlike in the case of solid components and panels, such as plastic panels and metal panels, on which threads can be formed easily, this is not possible in the case of a foamed sandwich panel owing to the foam core.
Two conventional methods for producing connecting points on sandwich panels are known in the prior art. One method relates to injection molding or foaming encapsulation of the connecting elements with the core layer material, for example a foam of the sandwich panel (in-molding method). Here, for example, metallic threaded inserts are already integrated into the sandwich component during the production process. To this end, however, the required inserts, such as the abovementioned threaded inserts, have to be inserted between the cover layers (carbon fiber reinforced plastic panels) manually or by means of industrial handling technology and subsequently have to be encapsulated by the foam material. The integrally joined connection which is produced in this way can subsequently be loaded mechanically.
Among other issues, the circumstance here is disadvantageous in that the position of the connecting points has to already be fixed during the production. Furthermore, shape and positional tolerances and the associated tolerance compensation provide further problems. If further or other comparable fastening points are required later, they can no longer be added. Furthermore, the connection between the inserts and the sandwich component takes place via an integrally joined connection to the foam during curing of the foam, as a result of which a simple replacement or repair is also not possible.
Furthermore, profiled strips and connecting strips are known from the prior art which are typically fastened to the outer side of the sandwich components by means of screws which penetrate the panels. Document WO 002010121830A3 is mentioned by way of example in this regard. This document discloses an elongate fastening element having a U-shaped strip piece, a profiled strip which is divided into two part elements, and a connecting device which makes it possible to fasten modules utilizing a clamping mechanism. A special solution of this type is unsuitable, however, for equipping sandwich panels with connecting possibilities.
A further type of production of connecting points which is known in the prior art is the application of what are known as “onserts” to the sandwich panels. “Onserts” are connecting elements which are designed specifically for adhesive bonding with an, as a rule, plate-like base part. Onserts exist in different configurations. It is advantageous in this method that weakening of the sandwich components can be avoided owing to a lack of machining such as drilling, riveting or the like. Secondly, the connections have the disadvantages which are typical for adhesively bonded connections, such as the lack of temperature resistance and strength fatigue which is dependent on the former. In the case of transverse loads, adhesively bonded connections tend toward what is known as peeling off, which is likewise disadvantageous and is associated with weakening of the strength of the connection. Furthermore, the adhesive bonding process is complicated and requires defined process and curing times before further machining can take place.
During the production of lightweight structures, it is also disadvantageous at various positions if components such as onserts project from the sandwich composite structure with threaded pins or threaded bushes.
It is therefore an object of the present invention to provide a connecting arrangement for a sandwich component, which connecting arrangement overcomes the abovementioned disadvantages. Furthermore, the arrangement should be capable of being used more universally and should produce a satisfactory mechanical connection to the sandwich component without adhesive. Furthermore, it is an object of the present invention to propose a method for producing a fastening possibility, as to how an arrangement of this type can be integrated onto or into the composite component in a simple and inexpensive way.
This object is achieved by way of a connecting arrangement and a method for producing a fastening structure in accordance with embodiment of the invention.
According to the invention, a clamping arrangement is provided having two interacting elements, in which the first element (holding element) is inserted into the sandwich component and the holding element is transferred into a “clamping” state with the sandwich component by way of actuation of a second element (fastening element). The free ends of fixing arms of the holding element which are provided for the clamping action are pressed against the core layer of the sandwich component under tensile loading, whereby tensile forces which occur as a result of a positively locking connection are conducted via the upper cover layer into the shaped part. In the context of the present invention, “clamping” is to be understood to mean a state, in which a defined section of the first component is pressed onto or partially into the core material in a non-positive and/or positively locking manner. Any suitable structure or any suitable material can be used as the core material, such as a foam, a metal foam on the basis of textile nonwovens, extrudable materials, multi-skin sheets, rib structures, honeycomb structures and the like. The penetration or insertion into the zone between the cover layers (i.e., the core zone) can be realized, for example, by way of a hook, a projection or a rib on the fastening element, which, in the clamping position, are pressed into the core material, for example the foam, or simply into the cavity in the region of the core zone. According to the invention, the holding element is equipped to this end with at least one fixing arm which can be actuated from a mounting position into a fixing position. The fixing arm can be actuated from its rest position (mounting position) into its engagement position with the core (fixing position) by way of pushing of the actuating element into the holding element.
A further aspect of the present invention relates to the geometric adaptation of the elements to one another. In a favorable case, the upper end of the fixing arm which is active for the non-positive connection is to be at a spacing of “zero” with respect to the upper cover layer and is therefore to bear against the cover layer with contact, with the result that, in the case of tensile loading, the tensile forces are introduced directly via the upper cover layer into the shaped part. At any rate, the spacing between the active end of the fixing arm and the upper cover layer is to be selected to be suitably small, with the result that other dimensions which are to be maintained in the interaction of the holding element and the actuating element do not become greater, whereby a stable firm fit can be achieved overall.
In the most general embodiment of the invention, a snap-together two-piece fastening device is provided for plug-in mounting into an opening of a sandwich component. The fastening device consists of a holding element with one or more fixing arms which can be actuated from a mounting position into a fixing position and an actuating element which can be plugged into an opening of the holding element. The actuating element deflects, in its position in which it is plugged into the holding element, at least one fixing arm into its fixing position. In the context of the present invention, two-piece device does not necessarily mean a complete separation of the two elements, but rather a functional separation. One part (holding element) serves for the holding function and the other part (actuating element) serves for transferring the first part from a mounting position into a holding position.
In one advantageous embodiment of the invention, the two snap-together elements (holding element and actuating element) have a substantially cylindrical (pot-like) or rectangular enveloping shape, it also being possible for a cylindrical shape with an elliptical base area to be used.
According to the invention, a fastening device is provided, furthermore, in which a plurality of actuable fixing arms are arranged on the preferably cylindrical holding element in a radially circumferential manner. In the context of the present invention, “actuable” is understood to mean a preferably elastic movement of the fixing arm from a mounting position into a fixing position, the movement from the mounting position into the fixing position taking place by way of active deflection by way of the actuating element.
In a further preferred embodiment of the invention, on their side which is directed from the center of the fastening device substantially toward the outside, the fixing arm or arms have engagement elements which are configured for mechanical penetration into a sandwich element when the fixing arms are actuated by the actuating element into their fixing position. In this position, the engagement elements of the fixing arms grip into the foam, with the result that the fastening device experiences a firm fit in the composite component.
A further aspect of the present invention relates to the configuration of the holding element. Since the aim of the present invention is the provision of a suitable fastening possibility on a composite component, the holding element is provided with a desired coupling form, such as an internal thread, a threaded pin, a latching mechanism or the like. Accordingly, the holding element advantageously has a central fastening element for connecting to an external fastening element.
The movable, preferably elastic fixing arms are advantageously fastened with their one end to the holding element, whereas the other end is configured as a free end. The connecting point of the fixing arm can thus act like an elastic hinge.
In one particularly preferred embodiment of the invention, at least one actuating arm is provided on the actuating element, which actuating arm, when the actuating element is plugged into the corresponding cavity or opening in the holding element, is guided with a control face along the fixing arm in such a way that the fixing arm is moved or elastically deflected in a direction away from the actuating arm toward the composite component. The actuating element locks the fixing arm in its completely pushed-in position, with the result that said fixing arm is prevented from returning into its original mounting position (starting position).
It is preferred if the actuating arm or arms have in each case one wedge-shaped form. By means of a structural configuration as described above, the fixing arm can be transferred continuously into its fixing position during the plug-in operation.
Furthermore, it is provided in one embodiment according to the invention of the fastening device that, in the snapped-together state of the actuating element to the holding element, the fastening arm or arms bear with their side face which points toward the center (Z) against the central fastening element and with the side face which points away from the center (Z) against the respectively correspondingly arranged fixing arm. A particularly effective firm fit in the composite component is achieved by way of this mutual contact of the functional components which are provided for fixing the holding element.
In one particularly advantageous embodiment of the invention, the fastening device is produced from plastic. As an alternative, the actuating element might also be configured from metal, for example as a high pressure die cast part, or from a combination of metal and plastic.
It is advantageous, furthermore, if respectively correspondingly arranged latching elements are provided on the holding element and on the actuating element, in order to latch the actuating element to the holding element in its position, in which it is plugged into the holding element.
A further aspect of the present invention relates to a method for producing an integrated fastening structure on or in a sandwich component with a fastening device as described above, having at least the following acts:                introducing a holding element of the above-described fastening device into an opening which is provided to this end in the sandwich component or introducing it directly into the sandwich component; and        actuating of the holding element by way of the actuating element, with the result that the fixing arms or arm of the holding element are/is actuated from their/its mounting position into their/its fixing position, in such a way that they/it penetrate/penetrates with at least one section which is provided for fixing into the core zone in the sandwich component.        
The method can be characterized by the following further acts:                producing an opening which is open on one side, such as a blind bore, in a sandwich component for introducing the holding element;        the introduction of the fastening device can take place using suitable methods, such as compression of the core material optionally in combination with fusing by way of friction or ultrasound, as a result of which a welded connection between a part of the fastening device and the sandwich component is possibly also produced as intended; and        introducing adhesive in order to produce an adhesively bonded connection, in particular in order to improve the transmission of force from the fastening device into the sandwich component.        
In the case of a suitable geometric condition for preventing a rotational movement between the sandwich component and the fastening device on one side and the holding element with the actuating element on the other side, it can be ensured even before the fixing position is achieved that the pre-mounted fastening element by way of tightening of a screw, for example using a cordless screwdriver, the “plugging in” and “fixing” is possible in one step by way of the translational movement between the holding element and the actuating element.
Furthermore, it is possible to achieve the clamping action in the end position by way of an annular material cutout, as a result of which a simple and robust embodiment is achieved.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.